If you noticed fewer blooms on your bearded irises this year, it's probably time to dig them up and divide them. / Bill Cary/The Journal News

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The Journal News

Like many long-lived, hardy perennials, bearded irises do best if you divide them every three to five years. If you don't, you'll begin to see fewer and fewer blooms each year, especially in the dead center of the clump.

Luckily, iris rhizomes like to grow right at the surface of the soil, so it's fairly easy to dig them up for division. And right now is a great time of year to do it.

"Most people don't realize that iris are dormant in the summer," says iris breeder and propagator John Schoustra of Greenwood Daylilies + Iris in Somis, Calif. "They generally need six to eight weeks of growth after the last bloom cycle before division. The exception is repeat bloomers - you can divide repeat bloomers anytime, as long as it has been six to eight weeks since the last bloom."

If you get newly divided irises into the ground now, they'll have plenty of time to settle in before winter. Well-established roots will keep them from being heaved out of the ground in next spring's freeze-thaw cycle.

In the Hudson Valley, tall bearded irises need at least six to eight hours of full sun each day. They also like well-drained soil so that the rhizomes never sit in water.

Avoid using mulch in a bed of irises. This just encourages funguses and prevents the rhizomes from absorbing the summer heat they need to bloom.

Schoustra says the magic soil amendment is gypsum. Iris like organic matter, so work in compost or amendments just as you would when planting a rose. An extra helping of gypsum will help release iron in the soil. It also helps leach out built-up salts from tap water.